We didn’t use it in our first few sessions but no sooner did we start taking on Large or bigger opponents did someone ask about flanking. Can’t help but think flanking is used so much because it’s been a staple of the game and was such as effective tactic for the past couple of editions that now we depend on it as part of our D&D approach to combat.

I've found in my games that flanking in 5e is too easy, largely due to the fact that creatures can freely run in circles around each other. I only allow flanking if the group agrees to change the rules about reach and opportunity attacks to be more like previous editions. But generally I find that I don't miss flanking! There are a lot more interesting/dramatic things to think about in combat.

We house ruled it into the game before we found the text in the book stating it was an optional rule. It turned out our house ruling was identical to the optional rule, so it meshed well. Based on the fact that we injected flank into the rules ourselves, that should be read as, "We like the flank rules."

A little surrounding information: We're all experienced gamers (multiple decades each, except for one player who isn't multiple decades old yet), so we tend to "make up" rules that make sense on the fly to prevent disruption to the game flow. Between sessions, we'll look things up and have Discord discussions on ways to improve our "on the fly" decisions. With the level of experience we all having in gaming, we do a pretty good job of extrapolating what parts of the book(s) we've read and remember into accurate rulings. Having said that, we get it wrong sometimes, too.

Question for those who do use the optional 5e flanking rule: do you run combats more tactically (on a grid with minis) or more theater of the mind? If the former, does your game involve counting squares and people spending time--even if its a few seconds each turn--determining how they get from point A to point B on the map? If the latter, do you just hand-wave tactical movement and say, "Yeah, you can run up and flank the ogre no problem."

Theatre of the mind for us and yeah we just say we do it so long as we’ve already established we’re within close range of the target. If you have to move to get close to it, you can’t move and flank on the same turn.

Here's another tangential but related question. Is advantage in 5e so powerful that getting it becomes more important in the mechanics than most anything else?

I guess that depends a bit on what the "anything else" might be (and it definitely depends on how good your chances already are without advantage), but rolling with advantage can get you the statistical equivalent of up to like a +5 on a regular roll for mid-range target numbers, so in the world of the 5E bounded ranges, that's pretty good!

Exactly, Rob! When your chance of hitting with a melee attack increased by ~25% simply because of where your character stands, that is significant. That gives getting INTO that position a very high mechanical value.

Here's another tangential but related question. Is advantage in 5e so powerful that getting it becomes more important in the mechanics than most anything else?

I do feel like advantage IS that powerful. It's a hefty bonus, and usually if you can get it for one attack in a turn you'll get it for any other attacks you make as well - and that makes a huge difference for anyone with Extra Attack. Not to mention feats like Great Weapon Master.